[OSM-newbies] Changes over time (demolished and abandoned buildings, felled trees)

[Nicholas Shanks]

On 23 Mar 2010, at 20:29, Someoneelse wrote:

> How to map stuff that isn't there any more has been discussed in the 
> past (unfortunately without any consensus being achieved as far as I recall)

Hmm, I looked at those. All of the ideas presented were a bit rubbish.

> If it was me, I'd at least mark the outline of e.g. the school with no 
> tags other than a note saying what it is.

Oh good, as that's exactly what I had done.

> I wouldn't try and tag it as 
> a school, because it isn't any more.

I can imagine some buildings going through several phases during their lifetimes, e.g from hotel to office block, to flats, to demolished. It seems like each tag would need it's own start and end dates!

So how about something like this:

timeline:1=bef. 1939
timeline:2=1945-6
timeline:3=ca. 1970
timeline:4=198203
period:1,4:amenity=townhall
period:2-3:amenity=school
period:1:name=OSM Village Hall
period:2-3:name=OSM Primary School
period:4:name=OSM Town Hall

If it's not clear, the timeline key defines temporal waypoints, which are used by the period key, which defines key/value pairs for specific points or spans of time. In my example, a building was definitely a town hall sometime before 1939, between 1945 or 1946 and about 1970, it was a school, and in March 1982 it was back to being a town hall again. Where a range of ID numbers is given (2-3) then the state of that tag is known to be continuously valid between those two (perhaps vague) dates. Where a single ID is given (4) then the state is only known on that date.

bef/aft/ca/bet = before, after, circa, between (between is implicit when given a range)
date format is YYYY[MM[DD]] (or perhaps YYYY[/MM[/DD]])
hyphens indicate ranges and do not separate YYYY from MM or DD

You would use "period:n:amenity=" with a blank value to clear the property (or if blank values aren't allowed, the keyword ‘no’).

There is only one problem I can forsee:
Inserting new temporal waypoints requires renumbering everything after it. perhaps take the BASIC approach and number everything in tens. (think: 10 PRINT "Hello World!" \n 20 GOTO 10). When new data is submitted, the waypoints should be automatically renumbered again to go up in tens, so that the next person to come along has the same breathing space. If a user needs to insert more than 9 temporal waypoints, he can hit save, then reload, and have the system automatically do it for him.

> I'd also add a source tag saying 
> where you got the info that there had been something there and how 
> accurate it is (for example in the case of the tree - an out of 
> copyright map could be quite a few metres out, but if you can poke a 
> spade in the ground and hit what remains of the stump it's as accurate 
> as your GPS is).


I just had the stump removed on friday, but the tree had been gone for 5 years.
But I don't have a GPS.

-- 
Nicholas.

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